Tryptophan is important both for protein synthesis and as a precursor
of niacin, serotonin and other metabolites. Tryptophan is an unusual a
mino acid because of the complexity of its metabolism, the variety and
importance of its metabolites, the number and diversity of the diseas
es it is involved in, and because of its use in purified form as a pha
rmacological agent. This review covers the metabolism of tryptophan, i
ts presence in the diet, the disorders associated with low tryptophan
levels due to low dietary intake, malabsorption, or high rates of meta
bolism, the therapeutic effects of tryptophan and the side effects of
tryptophan when it is used as a drug including eosinophilia myalgia sy
ndrome.